Java产生死锁的一个简单例子-追风

思路是创建两个字符串a和b，再创建两个线程A和B，让每个线程都用synchronized锁住字符串（A先锁a，再去锁b；B先锁b，再锁a），如果A锁住a，B锁住b，A就没办法锁住b，B也没办法锁住a，这时就陷入了死锁。直接贴代码：

	public class DeadLock {
		public static String obj1 = "obj1";
		public static String obj2 = "obj2";

		public static void main(String[] args) {
			Thread a = new Thread(new Lock1());
			Thread b = new Thread(new Lock2());
			a.start();
			b.start();
		}
	}

	class Lock1 implements Runnable {
		@Override
		public void run() {
			try {
				System.out.println("Lock1 running");
				while (true) {
					synchronized (DeadLock.obj1) {
						System.out.println("Lock1 lock obj1");
						Thread.sleep(3000);// 获取obj1后先等一会儿，让Lock2有足够的时间锁住obj2
						synchronized (DeadLock.obj2) {
							System.out.println("Lock1 lock obj2");
						}
					}
				}
			} catch (Exception e) {
				e.printStackTrace();
			}
		}
	}

	class Lock2 implements Runnable {
		@Override
		public void run() {
			try {
				System.out.println("Lock2 running");
				while (true) {
					synchronized (DeadLock.obj2) {
						System.out.println("Lock2 lock obj2");
						Thread.sleep(3000);
						synchronized (DeadLock.obj1) {
							System.out.println("Lock2 lock obj1");
						}
					}
				}
			} catch (Exception e) {
				e.printStackTrace();
			}
		}
	}

运行的结果如图所示：

可以看到，Lock1获取obj1，Lock2获取obj2，但是它们都没有办法再获取另外一个obj，因为它们都在等待对方先释放锁，这时就是死锁。

如果我们只运行Lock1呢？修改一下main函数，把线程b注释掉。

	public class DeadLock {
		public static String obj1 = "obj1";
		public static String obj2 = "obj2";

		public static void main(String[] args) {
			Thread a = new Thread(new Lock1());
			// Thread b = new Thread(new Lock2());
			a.start();
			// b.start();
		}
	}

	class Lock1 implements Runnable {
		@Override
		public void run() {
			try {
				System.out.println("Lock1 running");
				while (true) {
					synchronized (DeadLock.obj1) {
						System.out.println("Lock1 lock obj1");
						Thread.sleep(3000);
						synchronized (DeadLock.obj2) {
							System.out.println("Lock1 lock obj2");
						}
					}
				}
			} catch (Exception e) {
				e.printStackTrace();
			}
		}
	}

	class Lock2 implements Runnable {
		@Override
		public void run() {
			try {
				System.out.println("Lock2 running");
				while (true) {
					synchronized (DeadLock.obj2) {
						System.out.println("Lock2 lock obj2");
						Thread.sleep(3000);
						synchronized (DeadLock.obj1) {
							System.out.println("Lock2 lock obj1");
						}
					}
				}
			} catch (Exception e) {
				e.printStackTrace();
			}
		}
	}

运行结果为：

由于没有其它线程和Lock1争夺obj1和obj2，Lock1可以不断地循环获取并释放它们，这时没有死锁。